Battery voltages after full charge and after using inverter at night

SystemSystem Posts: 2,511 admin
Hey guys,
Ive got a small solar project right now, one Sharp 123 watt panel, a Morningstar charge controller and a Discover agm battery.

Currently, after the batteries fully charged I have 13.20 volts.

Now, my question is, how far do you guys usually drain your batteries voltage wise every night with your inverters? Ive heard theres a specific voltage you do not want to cycle below, and Im curious what that number is as well.

Thanks
Cory

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,800 admin
    Re: Battery voltages after full charge and after using inverter at night

    The problem is that monitoring battery voltage is not a great way to "know" when they have been discharged to XX% state of charge--it is not very accurate.

    Generally, you don't want to cycle below 50% State of Charge for long life, and don't cycle below 20% state of charge to avoid damage.

    You really are better off monitoring current * time so that you know when yo have reached XX% discharge of the battery's rated capacity (AGM batteries are sealed, so you cannot use a hydrometer to monitor their state of charge via specific gravity readings).

    A battery monitor is the usual tool--but they are not cheap. Especially for small systems.

    If this is a smaller system (single battery supplying a relatively small load)--you can use one of these Amp*Hour/Watt*Hour meters used by Radio Control guys. You can sort of make a battery monitor out of them (see FAQ's). For your education, a pretty good way to go.

    And of course, the Battery FAQ's:

    Deep Cycle Battery FAQ
    www.batteryfaq.org

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarusicarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Battery voltages after full charge and after using inverter at night

    Just to expand a bit on Bill's point. (Read the battery faqs for a more in depth discussion)

    Battery voltage is very dependent on loading. A fully charged 12 volt battery, at rest (neither being charged or discharged) will read ~ 12.6 volts. Add a load, such as a light bulb, and the voltage will drop, maybe as low as say 10 volts depending on how big the load is relative to the size of the battery. If you were to measure the 10 volts, you would say,, that battery is dead, when in fact is is fully charged. (or nearly so).

    On the other hand, if you had a severely discharged battery at it's at rest voltage was say 12.0, then you put it on a charger, and quite quickly the voltage would read ~12.6 you might be tempted to say,, this battery is charged when in fact it is still nearly discharged.

    The only real useful measure of battery voltage (as a tool to determine state of charge (soc)) is at rest for at least an hour, and ideally several hours.

    A much better measure of battery SOC is specific gravity, as measured by a hydrometer. That coupled with at rest voltage, as well as recent charge/load history will give you the best indication of SOC. A battery monitor, like the tri-metric will give you a pretty good, ongoing "gas gauge" of battery health,,, not perfect, but pretty close.

    Tony
  • bryanlbryanl Solar Expert Posts: 175 ✭✭✭✭✭
    Re: Battery voltages after full charge and after using inverter at night

    I understand that it goes against the religion here and what that entails ...

    But the fact is that you can do quite well in obtaining sufficient accuracy in smaller systems by measuring voltage. Just make sure that there is no significant charging or discharging for at least a half hour before you check the voltage. When it gets under 12.2v, you need to start thinking about getting it charged. When it gets down to 12.0 v, you need to put a battery charge on the high priority list.

    If you engage in regular habits, you can soon learn how to calibrate voltage measures to charge needs timing starting with this rule of thumb.

    In 'amateur' use, electron counters can convey an inappropriate sense of accuracy and need proper programming as well as calibration. They often do not fully compensate for temperature, cycle to cycle battery variation, use profiles and other such things each of which can have as much as a 20% impact on available battery capacity.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Battery voltages after full charge and after using inverter at night

    i agree with bryanl to a point as i use the voltage on my agm and is far greater in accuracy to do than to rely on some rough guess work for some ah meters. i do think it better to go out several hours rather than a half hour with no charge or discharge to get a good at rest voltage. now this will not be indicative as to the capacity itself of the battery as batteries can lose capacity, but it can still be used to show if it is at a certain point of the soc.
    also note that agm batteries have higher overall voltages than standard fla types as mine being fully charged and at rest for several hours will typically read around 12.8v and do note that it was reading higher when it was newer. i can't say what voltages are normal for your particular battery and an inquiry to the manufacturer is in order for better figures.
  • icarusicarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Battery voltages after full charge and after using inverter at night

    As I said in post # 3: "at rest voltage, as well as recent charge/load history will give you the best indication of SOC"

    Having a history, knowing how you have loaded/charged your system, coupled with volt meter readings will net/net give you a pretty good idea of SOC. Voltage alone, without the history won't.

    Tony
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